1. Field of the Invention
The present invention relates to a ferroelectric memory device. More particularly, the invention relates to a ferroelectric memory device having a group of memory cells that can be placed in a state in which the contents of the memory cells cannot be rewritten at a normal operating voltage. The invention also relates to a fabrication method for this type of ferroelectric memory device.
2. Description of the Related Art
Known ferroelectric memory devices are described in Japanese Unexamined Patent Application Publication No. 2001-94065 (hereinafter, Reference 1) and in ‘Preparation of Bi-Biased Ferroelectric Thin Films by Sol-Gel Method’ by Atsuki et al., Japan Journal of Applied Physics, Vol. 34, Part I, No. 9B (September 1995), pp. 5096–5099 (hereinafter, Reference 2).
Ferroelectric materials exhibit a charge polarization that is reversible by an electric field, and retain a fixed remanent polarization even after the electric field returns to zero. These properties are used in nonvolatile ferroelectric memories having ferroelectric materials as capacitor dielectrics.
Ferroelectric memories are generally used as read-write memories for storing variable data, such as user data, that may need to be altered from time to time. Devices such as microcontrollers, however, also require read-only memories for storing fixed data such as program code. It would therefore be desirable to have a ferroelectric memory including a read-only area.